1. Field of the Invention
The present invention relates to a novel process for producing packed tofu, particularly a process for producing long-life packed tofu from sterilized soybean milk.
2. Description of Related Art
Tofu (bean curd) is largely divided into two types: momen (pressed) tofu and silken tofu. Tofu is also divided according to the process of preparation into (1) cut tofu which is obtained by coagulating soybean milk in a mold and, after immersing in water, cutting to size and (2) packed tofu which is obtained by filling a small container of given size with soybean milk containing a coagulant, sealing the container, and allowing the soybean milk to coagulate in the container.
Conventional tofu generally keeps only 2 to 3 days at most even if kept at a temperature of 10.degree. C. or lower. In order to prolong the storage life of tofu, processes for producing sterile packed tofu from soybean milk having been sterilized at high temperature or retort tofu from non-sterilized soybean milk have been proposed.
The flavor of tofu is influenced by not only the quality of raw soybeans and water but also by the kind of the coagulants. In general, tofu prepared by using, as a coagulant, natural bittern, magnesium chloride, magnesium sulfate, calcium chloride, calcium primary phosphate, calcium sulfate, and calcium lactate, is more flavorful than that prepared by using glucono-.delta.-lactone. The natural bittern, magnesium chloride, magnesium sulfate, calcium chloride, calcium primary phosphate, calcium sulfate, and calcium lactate are generalized and referred to as "bittern" hereinafter. However, since bittern is generally quick to work, it causes partial coagulation on addition, and therefore it is difficult to use bittern in the production of packed tofu.
In the production of packed tofu, soybean milk to which the coagulant has been added is packed into a container and coagulated by heat. Uneven coagulation will occur unless the soybean milk containing the coagulant is rapidly packed into the container and heat-coagulated. Therefore, various manipulations would be required to avoid such uneven coagulation. Besides, the above-described bittern tastes bitter or astringent if added in a large amount, and tends to impair the taste of packed tofu, which is prepared by coagulating soybean milk in a container.
Where, in particular, long-life tofu is produced from soybean milk having been sterilized in high temperature, a larger amount of bittern is required for coagulation than used for general soybean milk because the gel-formation ability of soybean milk is reduced to 1/3 to 1/5 by the sterilization. In this case, the above-mentioned disadvantage of using bittern in packed tofu would be exaggerated.
Thus, it has been practically impossible to produce packed tofu with satisfactory palatability and texture by using sterilized soybean milk containing bittern as the main coagulant. At present, slow-acting glucono-.delta.-lactone has been used as a main coagulant in the production of long-life packed tofu.
Japanese Patent Laid-Open No. 241168/95 discloses a process for producing tofu aiming at energy saving, in which transglutaminase is added to sterilized soybean milk in as large an amount as 10 to 40 units per gram of protein so that the sterilized soybean milk can be coagulated at room temperature. However, the tofu obtained by this process is seriously different in texture from one prepared by using bittern as the main coagulant, as will be demonstrated in Test Examples hereinafter given. The difference in texture is attributed to the decided difference of transglutaminase from bittern in mode of binding reaction among protein molecules induced thereby.